Change seconds argument of make_*() functions to numeric

Along with the discussed change of the return type of EXTRACT from 
float8 to numeric [0], I was looking around what other date/time APIs 
might be using float arguments or return values.  The only thing left 
appears to be the functions make_time, make_timestamp, make_timestamptz, 
and make_interval, which take an argument specifying the seconds, which 
has type float8 right now.  I'm proposing the attached patch to change 
that to numeric.

Can we change the arguments, as proposed here, or do we need to add 
separate overloaded versions and leave the existing versions in place?

[0]: 
https://www.postgresql.org/message-id/flat/42b73d2d-da12-ba9f-570a-420e0cce19d9@phystech.edu